Look on the tire sidewall for the max rating. Actual pressure depends on load, tire size/type and number of plies. 4 ply ratings usually run at 35 psig max, 6 ply 45-50, 8 ply 60 and so on.
However, 4 ply 13.6 x 28 F1 (ag) rears on my Fords are rated 18 psig max. I have 3 different manufacturers involved in the 3 Ford's tires and all are the same size/plies and have the same max pressure rating. An F4 of that size in industrial probably would be 8-10 ply and rated 60 or better max....don't know, just speculating on the F4s.